The water is a great place for a living thing, since they depend on water to float their molecules and support their chemistry. Life evolved in the oceans and filled them wherever there was enough light to photosynthesize or food to eat. But there were niches going unused, up out of the water, on the bare land of the continents. How could the potential niches there be reached and filled?
The land environment would have had several significant differences from the water environment that would need to be adapted to:
- NOTHING WAS ALIVE UP THERE. For the first, pioneer organisms (this term is applied to the newcomers in any "new" environment), they needed to deal with an environment devoid of life and nutrient-poor. Animals might use it as a place to avoid predators, but would need to return to the water to feed. Plants had a trickier obstacle: the light, water, and carbon dioxide they needed for photosynthesis might be available, but other nutrients for making molecules such as proteins would not be. It is quite likely that plants would not have been able to move onto the land without symbioses established with fungi and bacteria to help them get the materials they needed.
- WATER EVAPORATES IN THE AIR. The water content of cells is critical to the function of cells - if too much is lost or gained, the cells cease to function. A land organism cannot lose too much water to the air or it won't survive. But there are transitional ecosystems that might have required adaptations usable against evaporation: tidal zones, where organisms are sometimes left "high and dry," as well as in pools that might fill with rain runoff or evaporate, where resisting a similar dilution change in the cells would be necessary; fresh water systems, where a resistance to the inflow from very dilute surroundings would be necessary. Our distant ancestors, the bony fish, apparently evolved in fresh water and developed an efficient waterproofing system to keep water from rushing into their cells; that barrier could also be used to prevent water loss in the air. It is quite likely that, for this and other reasons, all life on land evolved from tidal and/or fresh water ancestors. Of the three multicelled Kingdoms, the fungi seem to have had the hardest time with drying, perhaps because of the way nutrients get absorbed - it's almost impossible to move materials across a waterproofed surface - but they've gotten by in moister environments, in soils and in the wetness of other living things.
- YOU CAN'T FLOAT IN THE AIR. The buoyancy of water reduces the need for strong support structures. This was especially a problem for plants, which didn't undergo much dramatic evolution until they moved on to land, where complex support structures and then structures to move materials around against the force of gravity led to an explosion of different forms. Animals had some adaptations ready to go: muscle systems for moving quickly through the water or across the bottom needed modification to work on land (fins and fin supports needed to be more leg-like in our ancestors; insect and spider ancestors had to lighten their outer covering just to hold themselves up), but structures used for moving across tidal flats or in very shallow water became usable away from the water as well.
- TEMPERATURE FLUCTUATIONS. A body of water gains and loses heat more slowly than the air does, so temperature changes are slower there. Temperature has a huge effect on cellular chemistry, and only chemistry that can somehow deal with rapid changes can be used in a land organism. Again, tidal areas and shallow fresh water ecosystems would have been good staging areas for developing some flexibility. Plants, not being able to move from place to place to adjust their temperature, had a more critical problem, and may have taken some time to adapt to non-tropical areas.
- DIRECT SUNLIGHT. The frequencies of energy in sunlight can cause molecules in living systems to become unstable, as happens in the mutations that lead to human skin cancer. Water reflects several frequencies and quickly absorbs many more, making the problem much reduced for organisms that live below the surface. Most land organisms have protective pigments to keep the sunlight from penetrating and harming them. The adaptations would also have been required for life in tidal areas and shallow fresh water.
- MUCH MORE OXYGEN. As mentioned earlier, the air can hold much more oxygen than water can, and oxygen is a very reactive material (even you can be poisoned with too much of it!). An organism can't live in the air if it can't handle the increase in oxygen. Long-term, the higher oxygen levels allow for much more energetic metabolisms in aerobic animals. Even an animal like a crocodile gets such an energy advantage from breathing air that it would never evolve a water-breathing system again, and its difficult to understand how anyone could ever develop a system by which a human could breathe underwater - there just isn't enough oxygen available there.
- SPERM NEED WATER. Sexually- reproducing animals and plants had for the most part evolved systems where the sperm were released and had to get themselves to the waiting egg cell by swimming. This doesn't seem like much of a problem, but for a couple of the major land groups it was the most difficult one to solve - long after the difficulties of water loss, and support, and other land challenges were met, amphibians and ferns still require open water for reproduction.
Virtually every phylum of organisms was able to get a least a few species up onto land, although they all still have some water-living species as well. Some researchers hypothesize that the rise of land plants, with hard-to-break-down carbohydrate support structures, pulled more and more carbon from the environment. Less carbon available for aerobic respiration might have let more oxygen accumulate, setting up an environment for higher-metabolism, larger animals.